Two-component polyurethane coating compositions are coating compositions that crosslink by formation of urethane bonds as a result of the addition reaction between the hydroxyl groups of a hydroxyl-functional binder component and the free isocyanate groups of a polyisocyanate crosslinker component. The hydroxyl-functional binder component and the polyisocyanate crosslinker component are stored separately from one another before being mixed to form the two-component polyurethane coating composition. Typically, two-component polyurethane coating compositions are prepared just prior to their application by mixing a hydroxyl-functional binder component with a polyisocyanate crosslinker component. Typical mixing equipment for two-component coatings present in automotive OEM coating plants are static mixers such as, in particular, Kenics mixers.
In automotive OEM coating plants the substrates to be spray coated are supplied in succession to the spray application apparatus, for example, by using an automatic conveying apparatus, for example, a conveyor belt. The spray application itself is effected while the individual substrate to be spray coated and/or the spray application apparatus are in motion.
The spray application process in automotive OEM coating plants is distinguished by breaks or interruptions which do not only occur between two individual substrates, i.e. not only in the time period after the spray coating of a substrate has been finished and before that of the following substrate is started, but even in the course of the spray coating of an individual substrate. Such breaks or interruptions may happen either unintentionally or deliberately and they may differ in duration. For example, they may take 0.5 seconds to 15 minutes. During such breaks or interruptions no coating material is sprayed and, after the break has ended, spray coating is taken up again at that position on the substrate surface where it had been interrupted when the break began.
Whereas such positions on a substrate surface as described in the preceding paragraph cannot be visually perceived while a clear coat layer applied from a two-component polyurethane clear coat is still wet or uncured, it may happen that such positions turn up as unwanted variations in optical appearance, after the clear coat has been bake cured. Examples of such variations in optical appearance are in particular visually perceptible mattings. Such optical surface defects mean a need for reworking or repainting and lead to productivity losses. In this respect there is a desire for a more robust automotive OEM two-component polyurethane clear coating process which minimizes or even eliminates the occurrence of said unwanted optical surface defects and the reworking or repainting effort associated therewith.
It has been found that said desire can be satisfied by providing an automotive OEM clear coating process carried out with a two-component polyurethane clear coat composition prepared in a certain manner from certain components.